Tubular cradle for a motor vehicle

ABSTRACT

An integral tubular metal cradle (1) for a motor vehicle, comprising tubular central portion (2) with two arms (5, 6 or 7,8) extending from each end (3,4) thereof and having a substantially open, semi-tubular cross-section, the arms being at a dihedral angle of less than 180° at the point of connection to the central portion (2). The arms (5,6,7,8) provide a connection both to the vehicle bodywork and to either a front wheel wishbone or the rear axle of the vehicle. A method for making the integral tubular metal cradle is also provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a metal sub-frame for a motor vehicle and to amethod for producing such a sub-frame.

2. Description of the Related Art

What is intended by "motor vehicle" is any motorized four-wheeledvehicle, particularly utility vehicles and touring cars.

In this description and in this patent, what is meant by "sub-frame" isa structural component, generally made of metal, for a motor vehicle,providing a direct or indirect connection between, on the one hand, thebody of the vehicle and, on the other hand, either the front wheels orthe rear axle of said vehicle. A sub-frame of this kind has a centralregion consisting of a profile which needs to have sufficient transversestiffness that it can withstand the bending stresses created by themovement of the wheels, to each end of which central region there aresecured two profiled arms which, directly or indirectly and on each sideof this central region, provide a connection with the body and with afront wheel wishbone or the rear axle.

What is meant by "front wheel wishbone" in this patent is the componentconventionally known as the wishbone or any part which has a functionequivalent to that of a wishbone.

The front sub-frame of a motor vehicle thus generally has, at each ofthe two ends of the central region, means of connection to the body andmeans of connection to a front wheel wishbone. It also generally hasattachment means for a torque rod connected to the engine, for ananti-sway bar and for the steering rack of the vehicle. A frontsub-frame of this kind therefore needs to be strong enough that it canwithstand the bending forces generated by the movement of the wheels,and take up the engine torque and the forces coming from the anti-swaybar and the steering rack. Another function of this sub-frame is to dampand filter all of these forces and vibration and noise resulting fromthem, for the comfort of the passengers of the vehicle.

The rear sub-frame of a motor vehicle generally has, at each of the twoends of the central region, means of connection to the body and means ofconnection to the rear axle assembly. Its principal function is toprovide a second stage of filtering of the horizontal and verticalforces and of vibration and noise resulting therefrom, the first stageof filtering being performed by the rear axle assembly itself, thepurpose of this being further to improve the comfort of the passengers;this solution employing a rear sub-frame is quite expensive and ischiefly used in so-called top-of-the-range vehicles. This rear sub-framealso needs to be mechanically able to withstand the forces to befiltered, although these forces impose far less stress than in the caseof the front sub-frame.

Metal sub-frames of this kind are already known and consist of twopressed half shells welded together longitudinally. Each pressed halfshell has an open profiled central region with the transverse shape of aU, which is extended at each of its two ends by two open profiled armsin the shape of a U, the assembly having the longitudinal shape of an H.A longitudinal weld, which may or may not be continuous, connects theedges of the open U-profiles of the central region and of the arms ofeach half shell. The two closed profiled arms at each end of the centralregion, which consist of open profiled arms joined together by the weld,are used for securing the body, generally by two connecting points, andthe other for securing, at one or more connecting points, either thewishbone of a front wheel or the rear axle. The securing of otherelements may be envisaged, for example, in the case of the frontsub-frame, these might be elements such as the steering rack, the enginetorque rod and an anti-sway bar, these being secured via attachmentmeans.

A sub-frame of this kind displays a number of drawbacks. It is expensiveto manufacture because it requires two pressing operations starting fromshaped metal blanks, and this causes material to be wasted. For makingthe longitudinal weld it is also necessary to provide a strip of metalalong each half shell, which strip has no other use, and this makes thesub-frame heavier and increases its size. What is more, the weldconstitutes a region which is more vulnerable to the repeat stressesapplied to the sub-frame, particularly the bending stresses generated bythe movement of the wheels. Finally, the sub-frame thus produced is aclosed piece, and this makes it difficult for the previously-describedconnecting and attachment means to be secured in a convenient way thatcan withstand the stresses.

The invention has sought to overcome these drawbacks and to design ametal sub-frame which, while retaining enough rigidity that it can takeup the forces applied to it, allows the wasting of metal inmanufacturing it to be minimized, its weight to be reduced, which isvery favorable in the case of a vehicle, its compactness to beincreased, which makes the overall design of the vehicle easier, thewelded regions which are vulnerable to repeat stresses to be minimized,and finally, allows more robust securing of the connecting andattachment means.

BRIEF SUMMARY OF THE INVENTION

The sub-frame according to the invention has a one-piece tubular metalstructure, that is to say it consists of a single length of tube. As apreference, it is made of steel.

The sub-frame according to the invention has a tubular central regionextended at each of its two ends by two arms which, in the transversedirection, have approximately the cross section of an open half tube andform between them, where they leave the end of the central region, adihedral angle of less than 180°, the two arms of each end providing adirect or indirect connection on the one hand with the body and, on theother hand, with either the wishbone of a front wheel or the rear axleof the vehicle.

A sub-frame of this type which, according to the invention, is one-piecetubular, gives a weight saving on account of the good rigidity of thetubular cross section, particularly with regard to the bending forcesfrom the wheels. Its weldless one-piece design makes it very compact andless vulnerable to the repeat stresses. Finally, its open design offersa number of simple and robust ways of securing the connection andattachment means.

The central region preferably has a simple tubular section which is notnecessarily circular, allowing it to withstand the stresses imposed onthe sub-frame. It may be cylindrical, or have curvature between its twoends, that is to say that it may have a bend between its central partand each of its two ends, depending on the constructional features ofthe vehicle. It may advantageously have an expanded tubular crosssection between its two ends to increase its rigidity, that is to saythat it may have a larger cross section in its central part than at itstwo ends. If necessary, a more complicated but tubular profile formed ofa deformed tube, for example deformed into a U or into a Y, may be givento this central region in order to improve its resistance to certainstresses. The thickness of the tube in the central region is notnecessarily constant; it may vary in order to obtain greater rigidityonly where needed, thus making it possible to minimize the weight of thecomponent.

The four arms, two at each end of the central region, in the transversedirection have approximately the cross section of an open half tube, itbeing possible for the cross section to be uniform or otherwise alongthe entire length of the arms. As a preference, they have a U shapewhich gives them good mechanical strength. The two arms of each end ofthe central region are used to secure the body, preferably using a firstconnecting means, and to secure the wishbone of a front wheel, if thisis a front sub-frame, or the rear axle of the vehicle, if this is a rearsub-frame, preferably also by means of a second connecting means.

Advantageously, the first connecting means for the body acts at twopoints of the two arms of each end of the central region. The two armsof each end do not necessarily have the same length. Nor are theynecessarily symmetric with one another. The thickness of the half tubeis not necessarily constant along the length of the arms: it may varyalong the arm so as to have the best ability to withstand the actualdistribution of the forces.

The one-piece sub-frame thus formed of a tubular central region extendedby two arms which have approximately the cross section of an open halftube, preferably in the overall shape of an H, has a structure which,because it is open for the arms, greatly facilitates the securing of thebody, of the front wheel wishbone or of the rear axle, and the securing,if necessary, of attachment means for the steering rack, the enginetorque rod and an anti-sway bar. Compared with the known solutiondescribed at the beginning of the present description, the inventionoffers great flexibility to form a sub-frame allowing a wide variety ofconnecting and attachment means which can be incorporated into the openstructure, making the sub-frame stronger and more compact.

In particular, the connecting or attachment means advantageously have aprofiled part which fits into the profile of the open half tube of thearms, to which profile they are then joined by welding.

Advantageously, the first connecting means for the body is positioned orcomprises at least one component which is positioned at the end of oneof the arms of each end of the central region. In this case, the end ofthe corresponding arms is very advantageously given the shape of aflattened U, each leg of the U comprising an approximately central holefacing the hole in the other leg. A sleeve, with a diameter veryslightly smaller than that of the holes and a length which is longerthan the external width of the flattened U is placed transversely to theU inside said holes. The ends of this sleeve protrude from each leg andare secured to each of these legs by welding. This design allows abought-out elastic suspension component to be fitted directly,preferably to the end of the arm, and this gives a very simple and verycompact assembly.

When the stresses applied to the sub-frame according to the inventionare very high, particularly for vehicle front sub-frames, the two armsof each end may be connected by a reinforcing means. Various types ofreinforcing means can be used.

As a preference, the means of reinforcing each end is a pressed metalstrip welded inside the open half tubes of the two arms to close themcompletely or partially.

Other types of reinforcing means may be envisaged, for example oneshaving a pressed profile welded to the outside or inside of the openhalf tube of the two arms in order to close them completely orpartially. More complicated reinforcements may be designed, for exampleby closing the profile of open half tube of the arms locally bydeformation or by combining them with connecting or attachment meanssecured to the arms.

The fitting of reinforcing means does not in any way reduce the basicstrength qualities of the sub-frame according to the invention. This isbecause the welds on the reinforcing means are not along the main lineof action of the sub-frame which is the longitudinal axis joining thetwo wheels, and the welds are therefore not subject to the main bendingforces.

Nor does the fitting of reinforcing means detract from theopen-structure quality of the sub-frame according to the invention. Thisis because these reinforcements can be fitted only after the sub-framehas been equipped with the necessary connecting or attachment means.

The invention also relates to a method of producing a one-piece tubularmetal sub-frame for an automobile.

According to the invention, the method uses a single length of metaltube.

The tube is preferably made of steel. Its thickness and section are notnecessarily uniform.

The following operations are performed on this length of tube:

the central region is immobilized in shaped clamping jaws on a machinesuch as a press,

longitudinal cut is made in a first end of the length of tube in anapproximately diametral plane using known cutting means which are notdescribed, in order to obtain two open half tubes. The length of the cutis calculated as a function of the longer of the two arms of this end ofthe central region of the sub-frame to be manufactured,

in the same way, a longitudinal cut is made in the second end of thelength of tube in an approximately diametral plane using known cuttingmeans, the length of the cut being calculated as a function of thelonger of the two arms of this end of the central region of thesub-frame to be manufactured, in order to obtain two open half tubes,

the cuts in each end are advantageously made simultaneously in a singlestep,

next, the two arms of a first end of the central region are formed byparting and by deforming the two open half tubes of a first end of thelength of tube using a shaped die, the central region being immobilizedin the clamping jaws. The shaped die, moved by known means such as ahydraulic ram, advances toward this first end of the length of tube inone or more steps and parts and deforms the two open half tubes untilthey form between them, where they leave the central region, a dihedralangle of less than 180°,

in the same way, the two arms of the second end of the central regionare formed by parting and by deforming the two open half tubes of thesecond end of the length of tube using a shaped die in such a way thatthey form between them, where they leave the central region, a dihedralangle of less than 180°,

advantageously, the operations of parting and of deforming the two endsof the length of tube are carried out simultaneously and preferably in asingle step,

the last step consists in cutting the arms of each end to length usingknown cutting means.

When the parting angle is large, uniformly spaced notches are made byknown means which are not described, along the cut edges of the openhalf tubes, before the parting operation, so as to avoid cracking duringthe operation of shaping the arms. In this case, it is also possible forthe operation of parting the arms to be carried out in several stageswith differently-shaped dies in order to reach the desired shapegradually.

Highly advantageously, each end of the tube is shaped before cutting,this being done using known means which are not described, in order togive the arms better ability to withstand the mechanical stresses. As apreference, each end is given a rectangular shape with rounded corners,in order to obtain U-shaped arms. The central region may also be shapedto give it better strength or a shape that suits the geometry of thevehicle.

This shaping may take place before or after the steps described earlierof cutting and of shaping the arms. For example, the central region maybe given some curvature between its two ends, that is to say that thetwo ends of this central region are held still using a tool ofappropriate shape and that the metal is deformed to obtain a bendbetween the central part and each of the two ends of said centralregion. The central region can also be given a more complicateddeformed-tube profile, for example a U or Y-shaped profile.

The central region can also be expanded before or after the cutting andthe shaping of the arms. In order to shape the central region, use isvery advantageously made of the hydroforming technique, and this ispreferably done prior to the cutting and shaping of the arms. To dothis, the following operations are carried out in a press:

a) the two ends of the starting length of tube are immobilized inconventional clamping jaws,

b) the central region is trapped in jaws the shape of which is afunction of the expanded profile to be obtained,

c) the central region is expanded by injecting fluid at high pressurewhich plastically deforms the wall until it presses against the shapedjaws of operation b, it being possible for operations b) and c) to beconducted or not completely or partially simultaneously.

This technique is highly advantageous in the case of the sub-frameaccording to the invention because the ends of the length of tube whichare used to clamp it, and which are usually wasted when components areproduced by hydroforming, are in this case used to produce the arms,thus minimizing the wastage of materials.

The last operation in the method according to the invention consists inequipping the sub-frame with the connecting means for securing the bodyand the front wheel wishbone or the rear axle, this being done by knownassembly techniques; if necessary, the two arms of each end areconnected using reinforcing means, as has already been described;attachment means can also be provided, in the case of a front sub-framefor example.

Very advantageously, the end of an arm is modified into a U shape tomake it able to receive directly a bought-out elastic suspensioncomponent for connection to the body, and this gives a simple andcompact assembly; to achieve this, the following steps are carried out:

a) the end of the arm is flattened by flaring it by any known means sothat the legs of the U are close together

b) a circular hole is made right through each leg, approximately in thecenter

c) a tubular sleeve with a diameter slightly smaller than that of saidhole is introduced into this hole, and one end of the sleeve protrudesout of each leg

d) the sleeve is welded to each leg.

The example and figures below give a nonlimiting description of oneembodiment of the sub-frame and of the method according to theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a front sub-frame according to theinvention.

FIG. 2 is section 2--2 of FIG. 1.

FIG. 3 is section 3--3 of FIG. 1, on a larger scale.

FIG. 4 is the part view in the Direction of 4 of FIG. 1, on a largerscale.

FIG. 5 is section 5--5 of FIG. 1, on a larger scale.

FIG. 6 is a section of the starting length of tube of the methodaccording to the invention.

FIG. 7 is a view of the starting length of tube used for the methodaccording to the invention.

FIG. 8 is a diagrammatic view of the hydroforming phase of the methodaccording to the invention.

FIG. 9 is a diagrammatic view of the step of cutting the arms of themethod according to the invention.

FIG. 10 is a diagrammatic view of the step of parting the arms accordingto the invention, at the end of deformation.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 diagrammatically depicts a one-piece tubular front sub-frame 1according to the invention, which in this embodiment is H-shaped.

This component has a tubular central region 2 of circular cross sectionexpanded between its two ends 3, 4, that is to say that the diameter ofits central part is greater than that of its two ends. FIG. 2 shows thesection 40 on 2--2 of the central part of the expanded region, which islarger than the section 41, depicted in FIG. 6, of the starting tube.Each end 3, 4 of the central region 2 is extended by two arms 5, 6, 7, 8forming between them, where they leave the end 3, 4 of the centralregion 2, a dihedral angle B of about 90°, the angle B being marked onlyon the end 3 in order not to clutter the figure. The four arms 5, 6, 7,8 have the U-shape of an open half tube as can be seen at reference 43in FIG. 3.

The arms 5 and 7 above the axis X--X of the front sub-frame 1 are usedfor connection to the body of the vehicle. The end 9, 11 of each of thearms 5, 7 is flattened and flared and has a hole 13, 14 in its center.FIG. 4, which is the part view on 4 of FIG. 1, describes the detail ofthe end 11 of the arm 7, the walls 15 and 16 of which are brought closertogether by comparison with the walls 17, 18 of the rest of the arm 7. Atubular sleeve 20 of a bought-out elastic suspension component with adiameter very slightly smaller than that of the hole 14 and a lengththat exceeds the width between legs 15, 16 of the end 11, is placedtransversely to the end 11 inside the holes, one end of the sleeve 20protruding from each leg, securing being by welding 21 to each leg 15,16.

In the same way, as can be seen in FIG. 1, the sleeve 19 of a bought-outelastic suspension component is secured to the end 9 of the arm 5 in theway which has just been described.

The arms 6 and 8 below the axis X--X of the front sub-frame 1 are usedfor connection to the front wishbone of a wheel of the vehicle. Acomponent 22, 23 is secured to the end 10, 12 of each of the arms 6, 8.FIG. 5, which is the section on 5--5 of FIG. 1, describes the detail ofthe end 12 of the arm 8 wherein it can be seen that the component 23 isfitted into the end 12 and secured by a weld 24 to the two legs of thisend. The component 22 is secured to the end 10 of the arm 6 in the sameway. The top of the component 23, 22 respectively, is assembledelastically with one of the three attachment points of the front wheelwishbone.

A reinforcement 25, 26 consisting of a metal strip is fitted into andwelded inside the paired arms 5, 6 and 7, 8 respectively, to close thembetween their ends 9, 10 and 11, 12. FIG. 3, which is section 3--3 ofFIG. 1, shows the reinforcement 25 fitted into the profile 43 of the arm6 and secured thereto by the welds 27.

This front sub-frame 1 is also equipped with another component forconnection to the body, with another component for connection to thefront wheel wishbone, and with various attachment points to suit thedesign of the vehicle. These connecting components and attachment pointsare not depicted in order not to clutter FIG. 1.

FIG. 8 diagrammatically illustrates the first phase of the method ofproducing the sub-frame 1 according to the invention: this is theexpansion of the central region 2 by hydroforming. The starting point isa length of steel tube 42, FIG. 7, of circular section 41 depicted inFIG. 6, the two ends 28, 29 of which are immobilized in the clampingjaws 30 on a machine, not depicted, such as a press. The central region2 is trapped between its two ends 3, 4 in jaws 31 the shape of which isdesigned to obtain the expanded profile 40 of FIG. 2. The central region2 is expanded between its ends 3, 4 by injecting fluid at high pressurewhich plastically deforms the wall until it contacts the shaped jaws 31.

Next, having freed the length of tube 42, the two ends of this length oftube 42 on each side of the ends 3, 4 of the central region 2 are givena rectangular shape with rounded corners using conventional shapingmeans.

FIG. 9 diagrammatically depicts the next step in the method according tothe invention. On a machine, not depicted, such as a press, the centralregion 2 is immobilized between its ends 3 and 4 in shaped clamping jaws32 and a cut 33, 34 is made in each end of the starting length of tubesimultaneously, using known means which are not described. Each cut 33,34 which is in the diametral plane of the tube, extends as far as theend 3, 4 of the central region 2 and delimits two open half tubes 35,36, 37, 38.

FIG. 10 diagrammatically depicts the next step in the method accordingto the invention.

With the central region still immobilized by the jaws 32, the arms 5, 6,7, 8 are shaped by parting and deforming the open half tubes 35, 36, 37,38 using two shaped dies 39 moved by hydraulic rams, not depicted, whichadvance simultaneously and gradually toward the open half tubes 35, 36,37, 38 and part them until they form between them, where they leave thecentral region, a dihedral angle B of about 90°, the angle B beingdepicted only in the left-hand part in order not to clutter the figure.

Next, the arms 5, 6, 7, 8 are cut to the desired length in a known wayby means which are not described. To obtain the front sub-frame 1(FIG. 1) according to the invention, the following steps then need to becarried out using conventional means which are not described:

a) the end 9, 11 of each of the arms 5, 7 above the axis X--X isflattened and flared to move the legs of the U closer together

b) a circular hole 13, 14 is made right through each flattened end attheir center

c) a sleeve 19, 20 with a diameter slightly smaller than that of thehole 13, 14 and an end of which protrudes from each leg of the U isinserted into each hole 13, 14

d) the sleeve 19, 20 is welded to the legs of the U

e) a component 22, 23 for connecting with the front wishbone of a wheelis fitted into the end 10, 12 of each arm 6, 8 below the axis X--X

f) the component 22, 23 is welded to the end 10, 12 of each arm 6, 8

g) a reinforcement 25, 26 consisting of a metal strip is welded insidethe paired arms 5, 6 and 7, 8 to close them between their ends 9, 10 and11, 12.

Steps e) and f) may be carried out before step a).

Likewise, step g) may be carried out before step a) or between steps d)and e).

Numerous alternative forms of the embodiment described are possible anddo not depart from the scope of the invention, especially as far as theshape of the central region, the shape of the arms, the presence orabsence of reinforcing means, the shape of the reinforcing means, andthe way of producing the connection with the body and with either thefront wheel wishbone or the rear axle are concerned.

What is claimed is:
 1. A sub-frame for a motor vehicle, comprising a metal, one-piece, tubular structure, the sub-frame having a tubular central region with two opposed ends, the sub-frame having two arms extending from each of the two opposed ends of the tubular central region, the two arms having a cross-section substantially in a shape of an open half tube, the two arms extending from each of the opposed ends of the tubular central region defining a dihedral angle between the two arms of less than 180°, whereby the arms are adapted to provide a connection with a body of the motor vehicle or with either a wishbone of a front wheel of the motor vehicle in the case of a front sub-frame or with a rear axle of the motor vehicle in the case of a rear sub-frame.
 2. The sub-frame of claim 1, wherein the tubular central region has a curvature between a central portion and the two opposed ends of the tubular central region.
 3. The sub-frame of claim 1, wherein the tubular central region has a larger cross-section in a central portion than at the two opposed ends of the tubular central region.
 4. The sub-frame of claim 1, wherein the tubular central region has a U or Y shape.
 5. The sub-frame of claim 1, wherein a first connecting means is mounted on at least one arm at each of the two opposed ends of the tubular central region whereby the at least one arm is adapted to provide a connection with the body of the motor vehicle.
 6. The sub-frame of claim 5, wherein the first connecting means is positioned at the end of the at least one arm.
 7. The sub-frame of claim 5, wherein the first connecting means comprises at least one component which is positioned at the end of the at least one arm.
 8. The sub-frame of claim 6, wherein the end of the at least one arm has a shape in cross-section of a flattened U with opposed legs, each leg having a central hole facing the hole in the opposed leg.
 9. The sub-frame of claim 6, wherein the first connecting means comprises at least one elastic suspension component mounted on the end of the at least one arm.
 10. The sub-frame of claim 1, wherein a second connecting means is mounted on at least one arm at each of the two opposed ends of the tubular central region whereby the at least one arm is adapted to provide a connection with the front wheel wishbone or the rear axle.
 11. The sub-frame of claim 1, wherein the arms have a U-shape in cross section.
 12. The sub-frame of claim 1, wherein the arms are connected by a reinforcing means.
 13. The sub-frame of claim 12, wherein the reinforcing means is a pressed metal strip fitted and welded inside the open half tubes of the arms thus completely or partially closing the half tubes.
 14. The sub-frame of claim 1, wherein the metal is steel.
 15. The sub-frame of claim 1, wherein both arms at the same end of the tubular central region are connected by a reinforcing means. 